1. Field of the Invention
The invention concerns a method and a device for testing for tightness deformable containers which regain their original form through their inherent or an external restoring force, particularly plastic containers, whereby a negative pressure is generated in the container and a change in the negative pressure is determined.
2. Scope of the Prior Art
In the case of so-called returnable plastic containers, particularly beverage bottles, and unlike returnable glass bottles, internal pressure tests are necessary prior to a refilling, due to the thinness of their walls and the risk of embrittlement of the material due to ageing. The purpose of the internal pressure test is to detect holes and embrittlement cracks in the container wall.
Hitherto, this has been very elaborately effected by means of a rotary machine with a star wheel, each bottle being transported in a star wheel compartment where it is pressurized with a defined pressure, and pressure changes being measured and evaluated after a definite period of time or a definite distance by means of a differential pressure measuring device. This measuring device must be set on the bottle opening in an extremely precise and impervious manner. This is relatively easily achievable due to the defined guidance of the bottles by means of the star wheel.
A method of the type initially referred to is known from U.S. Pat. No. 3,762,213. In this case, the negative pressure in the container is generated in that a cylinder with a displaceable piston is set on the container opening and the piston is then retracted. Due to the negative pressure, the container is then suspended from the cylinder, and the period of time for which the container is suspended from the cylinder is used as a measure of the tightness of the container.
Known from U.S. Pat. No. 4 862 732 and DE-A-41 21 867 is a method for testing the tightness of containers closed with caps, in which an overpressure is generated in the container by deformation of the container. The bulging of the container cap is used as a measure of the generated overpressure.